Serum alkaline phosphatase (AP) is an important marker of cholestasis, a condition in which the normal flow of bile from the liver is interrupted. Cholestasis may result from physical obstruction of the biliary tree (extrahepatic cholestasis), or from poorly understood mechanisms in which bile flow is interrupted at the cellular or sub-cellular level (intrahepatic cholestasis). Irrespective of the type of cholestasis, AP activity in the liver and circulation are uniformly increased. As most of the AP in the liver is found on the canalicular membrane, it is not clear how the enzyme gets into the circulation. Two possible explanations exist: a secreted form of the enzyme might be synthesized de novo, or membrane-anchored AP may migrate along the intercellular space towards the sinusoidal membrane where it is cleaved by phospholipase C. These two theories are not mutually exclusive. This proposal aims at answering some of the questions related to AP synthesis and secretion by the liver. Specifically, the studies will focus on: 1. Whether a secreted form of AP is synthesized by the liver. 2. How the increased synthesis of AP is mediated. 3. The process of AP attachment to the microsomal membrane. The techniques that will be used include histochemical staining of liver sections affinity chromatography, in vitro transcription and translation, Northern blots, tissue culture and precursor labeling. The significance of the work lies in gaining an understanding into the process of AP synthesis as it relates to the liver in normal and diseased states.